The system is requested to create a crash dump. This option is supported only when rebooting, and it has no effect unless a dump device has previously been specified with dumpon(8).
Boot the specified kernel on the next system boot. If the kernel boots successfully, the default kernel will be booted on successive boots, this is a one-shot option. If the boot fails, the system will continue attempting to boot kernel until the boot process is interrupted and a valid kernel booted. This may change in the future.
The halt or reboot is not logged to the system log. This option is intended for applications such as shutdown(8), that call reboot or halt and log this themselves.
The file system cache is not flushed. This option should probably not be used.
The system will turn off the power if it can. If the power down action fails, the system will halt or reboot normally, depending on whether halt or reboot was called.
The system is halted or restarted quickly and ungracefully, and only the flushing of the file system cache is performed (if the -n option is not specified). This option should probably not be used.
The fasthalt and fastboot utilities are nothing more than aliases for the halt and reboot utilities.
Normally, the shutdown(8) utility is used when the system needs to be halted or restarted, giving users advance warning of their impending doom and cleanly terminating specific programs.